deflagrate

Deflagration is a mode of combustion in which a flame front propagates through an unburned mixture at subsonic speeds relative to the mixture. The reaction advances primarily by heat transfer and mass diffusion, rather than by a shock wave. This contrasts with detonation, where a self-sustaining shock front drives the reaction at supersonic speeds.

Deflagration can occur in gases, vapors, and finely dispersed dusts and is influenced by the properties of the reacting medium, including reactivity, pressure, temperature, and confinement. Propagation can be laminar or turbulent; turbulence usually increases the apparent burning rate but remains subsonic with respect to the unburned material. In confined spaces, deflagration can generate significant pressure and cause structural damage if venting is restricted.

A key related phenomenon is the potential transition from deflagration to detonation (DDT). Under certain conditions—such as high confinement, highly reactive mixtures, turbulence, and obstacles—the flame can accelerate and transform into a detonation, producing a shock-driven, much more destructive event.

Deflagration has important safety implications in industries handling flammable gases, vapors, and dusts, as well as in the storage and use of propellants and explosives. Prevention and mitigation strategies focus on controlling ignition sources, limiting confinement, ensuring adequate venting, and designing processes to suppress rapid flame acceleration.

Etymology derives from the Latin deflagrare, to burn down or burn away. In practice, deflagration describes common, subsonic flame propagation distinct from the more energetic detonation.